The present invention relates to a motor to be used for information, audiovisual, industrial equipment, and the like.
Recently, higher-density recording is being promoted in the motors used for information, audiovisual equipment, etc. as represented by a digital video disk drive (DVD) unit and a hard disk drive (HDD) unit. With these advances, the motors used for such equipment are required to have higher rotational accuracy. There is also a growing demand for motors used in the machinery for manufacturing such equipment to have rotation accuracy high enough to meet the rotational accuracy of the equipment.
The causes of deteriorating the rotational accuracy of a motor are: (1) cogging torque resulting from a change in magnetic attraction force between the core and the magnet of a motor; (2) torque ripples produced from the current flowing; and (3) irregular vibration resulting from the shaft whiling with some deviation in the bearing. Among these causes, this invention particularly addresses reduction of cogging torque.
Conventional techniques of reducing cogging torque by specifically designing plane configurations of cores are disclosed in Japanese Patent Application Non-examined Publication No.H04-304151, Japanese Patent Application Non-examined Publication No.H09-163649, Japanese Patent Application Non-examined Publication No.H09-285047, and others.
The motor structure described in the above-mentioned Japanese Patent Application Non-examined Publication No.H04-304151 is shown in FIG. 52. In FIG. 52, the core reduces cogging torque by providing a positional relation with slightly different angles between salient pole tips 311, 312, 321, 322, 331, 332 and respective poles of magnet 302.
Meanwhile, techniques of reducing cogging torque by changing axial configurations instead of plane configurations of cores are disclosed in Japanese Patent Application Non-examined Publication No.H02-254954 and Japanese Patent Application Non-examined Publication No.H03-3622.
The motor structure described in the above-mentioned Japanese Patent Application Non-examined Publication No.H02-254954 is shown in FIG. 53.
In FIG. 53, cylindrical core 401 is divided into upper core 411 and lower core 412. The positional relation between upper core 411 and the magnet is different from that between lower core 412 and the magnet. This structure cancels out the cogging torque waveforms generated by the upper and lower cores each other, thereby reducing the cogging torque of the entire motor.
FIG. 54 shows the structure of a motor armature described in Japanese Patent Application Non-examined Publication No.H03-3622.
In FIG. 54, laminated core 501 is configured by axially laminating cores in which salient poles have different opening angles (an angle a salient pole tip forms with respect to the center of the core), or the salient poles of teeth have different widths at X and Y This structure allows different cogging torque waveforms to be canceled, thereby reducing cogging torque.
Techniques of reducing cogging torque using specifically designed magnet polarization instead of core shapes are disclosed in Japanese Patent Publication No.2588661. A structure of a brushless motor described in the publication is shown FIG. 55.
In FIG. 55, the motor is made in 4:3 structure in which ring-shaped magnet 602 has 4 n poles and the stator core has 3 n poles. Skew angle xcex82 of the magnetic poles of rotor magnet 602 is set as (30xc2x0/n)xc3x970.8xe2x89xa6xcex82xe2x89xa6(30xc2x0/n)xc3x971.2. This structure reduces cogging torque and distortion ratio of induced voltages.
However, these conventional techniques have the following problems.
First, among the techniques mentioned above, those utilizing core shapes have not completely eliminated cogging torque even though produced a certain extent of effects. Consequently, a level of cogging torque remains producing at one-half the cycle of basic cogging torque determined by a least common multiple of the number of core slots and the number of field poles.
Basic cogging torque may be calculated, for example, by determining the least common multiple (LCM) of the number of magnetic poles and the number of core slots in the motor. The LCM is divided into 360 degrees to calculate a basic cogging torque cycle. For 8 poles and 6 slots, for example, the basic cogging torque cycle is 15 degrees ((360 degrees/LCM 24)=15 degrees).
Meanwhile, for conventional techniques of providing a magnet with a skew, a large skew angle is needed for effective reduction of cogging torque. This generates more ineffective magnetic flux; thus involving such adverse effects in performance as decreasing motor efficiency and increasing core loss. Furthermore, as considerably affecting accuracy in polarization or motor assembling, the conventional techniques have posed problems such as unstable motor characteristics.
The present invention reduces the cycle of the cogging torque produced due to a basic configuration of a core to one-quarter or less of the cycle of basic cogging torque determined by a least common multiple of the number of core salient poles and the number of field poles, and to minimize the absolute value of the cogging torque as well.
The motor core of the present invention has the following structure.
A core used in a motor having magnetic field generating means having N and S magnetic poles for generating a magnetic field and the core made of magnetic material and opposed to the magnetic field generating means, where one of the magnetic field generating means and the core rotates with respect to the other:
in which the number of magnetic poles is 2 m and the number of core slots is 6 n (m and n are integers), and
in which a basic configuration of the core is determined by setting its slot opening angles (where xe2x80x9cslot opening angle xe2x80x9d is an angle xcex1 slot opening forms with respect to the center of the core) to a value ranging from 80xc2x0 to 95xc2x0 in electrical angle xcex1 (each corresponding to (a/m)xc2x0 in mechanical angle) and from 20xc2x0 to 35xc2x0 in electrical angle xcex2(each corresponding to (xcex2/m)xc2x0 in mechanical angle). This configuration allows the cycle of produced cogging torque to be reduced to one-half the cycle of basic cogging torque.
Electrical angle is defined in relationship to that portion of the core occupied by a pair of N and S magnets. One N and S magnet pair are assumed to occupy an electrical angle of 360 degrees. Thus, for example, if a slot occupies one half of the circumference occupied by an N and S magnet pair, then the slot is said to have an electrical angle of 180 degrees.
In addition, two core shapes are combined so that the slots in each core are displaced by an angle equal to one-quarter the cycle of the basic cogging torque ((90/k)xc2x0 in mechanical angle [k is a least multiple of 2 m and 6 n]). This cancels different cogging torque waveforms in the same motor, thereby reducing the resultant cogging torque cycle to one-quarter the cycle of the basic cogging torque and also minimizing the absolute value of the cogging torque.
Moreover, by polarizing the core with a skew angle equal to one-half or less the angle used for conventional techniques, the cogging torque can be further reduced while decrease in motor efficiency is minimized.